Composite articles manufactured from fiber reinforced thermoset resins frequently have some degree of porosity. Low to moderate performance composites with high resin to fiber ratios (e.g. 50% or more resin), such as sheet molding compounds will cure with small bubbles entrained throughout the article. Because cured resin surrounds these bubbles in the interior of the matrix, they usually do not cause performance problems. However, at the surface the bubbles are exposed. In industries such as automobile manufacture, articles formulated from sheet molding compound are frequently used in areas where they must be painted. This can be very difficult because surface porosities can blow unsightly holes in the paint finish during a paint bake cycle.
Much higher performance composite articles, such as wound fiber reinforced piping or pressure vessels for high pressure gas or chemicals, engines for solid fuel rockets and military hardware or ammunition, utilize very high fiber contents (typically as much as 85%). In such low resin content articles, the presence of porosity may provide significant leak paths for gases or liquids, especially corrosive hot gases such as those provided by an ignited rocket engine. The consequence of a leak during use of such articles can, of course, be catastrophic. It is therefore been proposed to seal porosities in cured fiber reinforced composite materials with an anerobic curing impregnant. Such impregnants are frequently used to seal porous metal parts such as metal castings and sintered metal articles. Descriptions of such impregnants may be found in U.S. Pat. Nos. 3,672,942; 3,969,552; 4,069,378 and EP No. 0101367.
In the impregnation of metal parts, the anaerobic sealant will usually contain a copper salt or complex at a level in excess of 1 ppm, typically 3-10 ppm as an accelerator of anaerobic cure. Copper at these levels in conjunction with the hydroperoxide catalyst and, usually, a saccharin coaccelerator, will give the sealant an unaerated pot-life, at ambient temperatures, of about 3 minutes to about an hour, which is usually sufficient to permit a vacuum or pressure cycle to force the sealant into the porosities of the article to be completed and aeration resumed before the pot begins to gel.
In commercial impregnation processes for small metal parts and the like, however, it is often the case that substantial quantities of moisture will be introduced into the sealant vat. This may come from condensation when refrigerated tanks are opened to air on humid days, from parts baskets which have been dipped in aqueous rinse baths, etc. It has been found that typical anaerobic impregnants which include copper salt accelerators can be severely inhibited by water. Although the repeated vacuum cycles to which an impregnation bath is subjected will help to get rid of water contamination, it would be desireable to have an anerobic acceleration system which is less sensitive to moisture since use levels vary and less frequently evacuated baths may accumulate high moistures of wet level between uses. Also, if pressure rather than vacuum is used in the impregnation process there is no mechanism for removing water.
Furthermore, when composite materials are impregnated, it is often more difficult to force the sealant into the article and consequently a longer pot life sealant is desirable. Also, depending on the article, it may be desirable to minimize catalyst levels so as to avoid undesirable reactions due to residual oxidizing catalyst in the composite. However, when copper content is dropped below the 1 ppm level, it has been found that the cure sensitivity to moisture becomes critical. Any significant moisture content can prevent curing altogether even in the microscopic porosities of the impregnated article.
It has been found that many composite articles have a tendency to absorb substantial quantities of moisture (as much as several weight %) from the atmosphere. This is believed to be due to the presence of hydrophilic groups, such as hydroxyl, amine or amide in the cured resin (typically polyepoxides or polyamides), or in reinforcing fibers such as Kevlar.RTM..
Accordingly, there is a need for an anaerobic sealant which will reliably cure upon deprivation of oxygen even in the presence of moisture. In particular, it is desireable that such a sealant be capable of curing in the microscopic porosities of a thermoset resin/fiber composite article at or near ambient temperatures and have a copper content below .about.1 ppm.